The visual pigments in single rods and cones of vertebrates will be studied using two microspectrophotometers, the dichromic microspectrophotometer DMSP which can measure absorbance, and linear and circular dichroism in individual receptors, and the photon counting microspectrophotometer (PMSP) which is designed to give the best possible signal to noise ratio, and which is sufficiently rapid and convenient to operate so that data can be obtained from many receptors in a short period of time. Studies are also being carried out on the ionic fluxes in and out of the receptors in light and darkness. The aims of this research are the following: 1. To measure the rod and cone pigments of many vertebrate species and to relate the pigment absorptions and the arrangement of different receptor types on the retina to the animals' behavior and choice of habitat (although current work concentrate on fishes, mammals will be studied later). 2. To study the configuration of the pigment molecules in disc membranes of the rod outer segments and the changes that occur after exposure to light and various reagents. 3. To learn more about the transduction processes by determining ionic fluxes through the receptor membranes. 4. To relate the electrical responses obtained through recording through micropipettes to the internal environment of the receptors (pH and ion concentrations) by measuring the changes in absorption of indicator dyes in the MSP which occur in light and darkness.